Dr Martin Kidman, Sick (UK) Product Specialist for Machinery Safety and a qualified Functional Safety Engineer, discusses AGV/AGC protective fields and the impact they have on productivity.
Developments in the safety laser scanner have been central to improving the workflow efficiency and productivity of Automatic Guided Vehicles (AGVs) and Carts (AGCs), enabling safe non-contact detection of personnel and obstructions when travelling through complex production layouts. They have enabled mobile vehicles to move more nimbly, freely and at greater speeds.
Getting the best output efficiency from AGVs/AGCs in modern production environments depends how well they can overcome the space restrictions of modern factory and warehouse environments, as well as working in closer proximity to each other.
One important development has been the ability for safety laser scanners to scan longer protective fields. The maximum protective field that a scanner can facilitate is a key consideration if you design and build AGVs and AGCs – or if you want to purchase one. However, field size alone should not be the deciding factor.
Achieving a smaller protective field footprint with more rapid system safety response times in an integrated safety system can be a more productive means of achieving the required safety in a smaller footprint.
Safe motion control
Using the time-of-flight measurement principle, a safety laser scanner measures radial distances in a horizontal or vertical plane. When integrated with encoders on an AGV, speed can also be measured, so that safe motion control can be implemented, and controlled deceleration and acceleration can occur, rather than emergency stopping.
Multiple field configurations on a scanner enable a system to recognise permanent objects such as machinery, fences and docking areas, as well as people and other AGVs. However, the warning field must be large enough to ensure the vehicle is halted before a person or object is reached.
The response times of the safety devices and associated control system are also very important values to consider; not taking care to ensure these values are reasonable can have a huge impact on an application.
A safety laser scanner is a Type 3 device as set out in the harmonised standard EN 61496-1. When a safety function includes a Type 3 safety laser scanner, the maximum performance level (PL) or Safety Integrity Level (SIL) that can be achieved is PLd (EN ISO 13849) or SIL2 (IEC 62061). There is also a Technical Specification for safety laser scanners, IEC 61496-3, which covers the particular requirements for Active Opto-Electric Protective Devices responsive to Diffuse Reflection (AOPDDR).
Types of field
There are four different types of field that can be configured on a modern safety laser scanner (see below).
Fields are created using associated software and fields can be combined to create a Field Set. Any combination of fields can be used in a field set, as shown below.
Safety laser scanners now have the facility to create many field sets, and a field set is chosen based on a set of input conditions (encoders, software-based, etc) which form a Monitoring Case.
However, switching between monitoring cases can add to the response time of a system, so the use of simultaneous field evaluation capabilities (such as looking at six protective fields in one field set) can be advantageous.
By default, a safety laser scanner must see an object twice (two revolutions) to give a positive identification but, in difficult environments such as where dirt, dust, welding sparks or heavy vibration are present, multiple sampling may be a useful option to ensure reliability. This increases the number of times an object must be scanned before a detection signal is created, but this adds further to the response time of the device.
There is an important trade-off between the need for multiple sampling and the total response time, as the response time is increased for every additional scan.
Safe AGV/AGC control
EN ISO 13855 is a harmonised standard dealing with the positioning of safeguards with respect to the approach speeds of parts of the human body and assists in the creation of protective fields, creating the safety footprint of an AGV/AGC. As the protective field length (SL) increases, so does the footprint because the scanner needs to see further ahead to compensate for its reaction time. It is therefore advantageous to keep SL as small as possible so that:
- AGVs can work in closer proximity to each other
- AGVs do not need large clearance spaces, either at the side or in front
- Stoppages or speed reductions due to objects and people in the vicinity of AGVs are reduced
- AGVs can move faster with smaller fields and increase productivity
Usually the largest value, and the one that varies the most, is the stopping distance of the vehicle, or SA. The stopping distance comprises the braking distance for the vehicle and the distance covered during the combined response time of the safety laser scanner and the safety control system.
Choosing a safety laser scanner with high reliability keeps the need for multiple scanning to a minimum and has a direct influence on the total response time of the system. This can make a significant difference in the total protective field length needed.
The transmission times to the control system can also be improved by hard-wiring OSSDs to inputs on safety controllers or by using a safe communication method. Using simultaneous field evaluation to monitor multiple protective fields and not having to switch field sets also minimises the response time of a system.
High-definition distance measurement
Sick's microScan3 Safety Laser Scanner,pictured right, uses safeHDDMTM (safe High Definition Distance Measurement) technology to deliver a high-resolution, digital method for safety-related time and distance measurement. The patented algorithms used for safeHDDM technology feature multi-signal evaluation to achieve a level of measurement reliability never before seen in safety scanners. The multi-pulse concept of safeHDDMTM generates approximately 80,000 individual pulses per scan - compared to some 500 generated with conventional technology. These 80,000 individual pulses are then used to create measured values.
Every safeHDDMTM measured 'value' is not just composed of a single time-of-flight measurement, but now includes evaluated information from 140 pulses. The digitised echoes are compiled into data packages that overlap during evaluation. This guarantees a significantly more stable time and distance measurement. Additionally, the pulse frequency is almost 170 times greater in relation to previous measurement procedures, plus the laser pulses themselves are also coded by a time delay of a few nanoseconds.
safeHDDMTM technology delivers the following benefits:
- Highly stable measured values are obtained for increased detection reliability
- A fourfold reduction in sensitivity to sunlight and ambient light
- Increased resistance to dust and deposit formation
- Virtually no mutual interference between devices
This represents a huge increase in availability for machines and vehicles compared with previously-available devices.
Follow the link for more information about Sick's MicroScan 3 Safety Laser Scanner, the Sick Flexi Soft safety controller, DFS60S safety encoders or any of Sick's machinery safety devices and systems.